Theoretical and Applied Genetics 39, 379--381 (1969)
G a m e t e S e l e c t i o n w i t h a n I n b r e d Tester 1 M. Z. EL-HIFNY, M. S. AHMAD, J. D. SMITH and A. J. BOCKHOLT Texas A & M University, College Station, Texas Summary. A common inbred tester was used to evaluate gametes selected from three complex hybrid populations using two different inbreds as elite lines. The results support STADLER'Scontention that gamete selection is an efficient method for extracting superior gene combinations from hybrid populations. The inbred tester increases the resolving power of the method by eliminating extraneous genetic variability and by providing a homogeneous check population for comparative purposes.
Introduction The initial successes of hybrid corn breeding were obtained with inbred lines t h a t were isolated directly from open-pollinated varieties. As the hybrid corn program developed emphasis was shifted from the isolation of new inbreds to the improvement of existing inbreds. STADLER (1945a, b) proposed a modification of early testing, which he called gamete selection, as a method of obtaining improved inbred lines. The scheme involves crossing, a genetically variable population with an 'elite' inbred line and subsequently crossing individual F 1 plants to a common tester. If an inbred tester is used, as is suggested in this paper, all test-cross progenies receive essentially identical genetic contributions from the tester and from the elite inbred. Thus, genetic differences among test-cross progenies result only from differences among the gametes sampled from the variable gametic source. This allows recognition of the better gametes, but it does not permit their intact recovery. However, if the test-cross progenies are compared with the elite inbred b y tester hybrid, those which are superior to this check hybrid are presumed to have received a gamete from the variable source which was superior to the gamete contributed b y the elite inbred. Inbreeding these F 1 plants would produce modifications of the elite line which would be superior to the original inbred. STADLER suggested that gametic sampling offers a more efficient means of sampling open-pollinated varieties. PINNELL et al. (1952) used gamete selection in improving inbreds and used single crosses as specific combining ability testers. The agreement for combining ability of S O and S 1 lines was very good indicating gamete selection was an effective breeding method. LONNQUIST and MCGILL (1954) studied gametic sampling as a means of obtaining better inbred lines for specific combinations. They concluded from the Technical Article No. TA7973, Texas Agricultural Experiment Station, College Station, Texas. Project No. 1280. Parts of dissertations submitted by the first two authors in partial fulfilhnent of requirements for Ph.D. degrees.
S O and S 1 test-cross results that selection of better varietal plants as sources of germplasm to improve the inbred lines effectively increased the probability of isolation of the superior gamete combination. GIESBRECHT (t964) pointed out in his s t u d y of gametic selection that superior gametes existed in open-pollinated varieties. But he failed to observe a relationship between the yields of the F 1 plants and those of their respective F 2 and F~ progenies. He used a single cross (WD• as a tester for his study. This paper sulmnarizes the effectiveness of gamete selection, using an inbred line as tester, in two related experiments with two different elite lines using the same sources of gametes.
Materials and Methods The following complex hybrid populations were used as sources of superior gametes: A. (FYD-2 • YS-2) • Indian Chief B. (FYD-2 • YS-2) x (Jarvis • Indian Chief) C. (FYD-2 X YS-2) x Jarvis Ferguson's Yellow Dent (FYD) and Yellow Surcropper (YS) are open-pollinated Texas varieties. The FYD-2 and YS-2 parental materials are from a second cycle reciprocal recurrent selection program. Jarvis and Indian Chief are open-pollinated varieties from North Carolina. The inbred lines Tx403 and Tx127C were used as elite lines. Tx403 is an inbred selected from the backcross (Tx203 • WF9) • Tx203. The inbred line TxI27C was developed from Mosshart Yellow Dent in Texas, and it has good combining ability. However, it also has numerous agronomic defects. The inbred line TxS08, a selection from Tx325 • B37, was used as a tester for both elite lines. This tester possesses good agronomic characters and combining ability. Bulked pollen samples were collected from each of the complex hybrid populations and used to pollinate the elite lines Tx403 and TxI27C. Thus, individual F 1 plants would differ according to the genetic complement of the gamete they received from the variable gametic sources. Visually selected F1 plants were selfed, backcrossed to four inbred parent plants and test-crossed to four tester plants. The numbers of test-crossed selections from populations A, B and C were 278, t78 and t95, and 27t, 284 and t 79 for the elite lines Tx403 and Txt 27C, respectively. In t 967 the evaluation tests for the gametic sampling using Tx403 were separated from those using Tx127C. However, both were grown in the same field. A "restricted" randomized block design having a nested subclassi-
380
~[. Z. EL-HIFNY, IV[. S. AHMAD, J. D. SMITH and A.
80
,
-----
,
~
,
-i
\ 80
90
100
110
120
130
lZ,0
field (bu./A) lor 7x127C lest
80
I
I
I
t
r
I
I
Range
PopulationA PopulationB PopulationC Total
bu./acre
No.
t28.1 --130.0 t30.1 --132.0 132.1 --134.0 134.1--136.0 136.t-138.0 150 138.1 -- 140.0 140.1--142.0 142.1 -- 144.0 t44.1--146.0
5 4 3 4 t 0 o 0 t
%
No.
t.80 t.44 1.08 1.44 0.36 0.00 o.oo 0.00 0.36
2 5 1 5 2 0 2 0 o
%
No.
1.01 2.53 0.51 2.53 1.01 0.00 t.ol 0.00 0.00
5 4 3 2 1 0 t 0 0
% 2.56 2.05 t.54 t.03 0.51 0.00 o.51 0.00 0.00
No. 12 13 7 tl 4 0 3 0 t
% t.79 t.94 1.04 t.64 0.60 0.00 0.45 0.00 0.t5
Total superior 18 i7 t6 5t Total selected 278 189 195 671 % superior 6.47 8.59 8.21 7.60 x Superior progenies were defined as those which differed significantly from the mean of the Tx403 • Tx508 check (~ = 118.8 bu./A.) at the .05 probability level
Test-cross Progenies:Y=1152 A 1x403, Tx508 :~=118.8 J'X
00
Theoret. Appl. Genetics
the three variable gametic sources/or the elite inbred line Tx4o3
i
' ! 70
BOCKHOLT:
Table 1. Frequency distributions o/ superior progenies 1 selected from
l
lest-cross Progenies : Y=120.3 fx127C , fx508:7=1127
J.
Table 2. Frequency distributions o/superior progenies x selected from 70
06'0
the three variable gametic sources/or the elite inbred line Tx127C 80
90 100 110 120 'field (bu/A.) for [x403 Test
130
140
150
lZig. 1. Frequency distributions for grain yield of the check populations and of the test-cross progenies of gametes sampled from the [(FYD-2 • YS-2) • Indian Chief] population using Tx127C and Tx403 as elite lines lOO
I
I
I
]
I
Test-cross Progenies:Y=121.3 -
80
50
~
PopulationA P9pulationB PopulationC Total
bu./acre
No.
122.6--125.0 125.t --127.5 127.6--130.0 130.1--132.5 132.6--t35.0 135.1--137.5 137.6--140.0 t40.1--142.5 142.6--145.0
22 9 14 6 7 0 1 0 0
%
No.
8.t2 3.32 5.17 2.21 2.58 0.00 0.37 0.00 0.00
18 t4 7 2 5 3 1 1 0
20--70
80
[
I
90 100 110 120 Yield { bu./A) for Tx127C Test
130
140
150
I
I
Test-cross Progenies: 7= 114.5 fx403./'x508 : ~-= 118.8
60
A
40
/J \
20 0
r
60
No.
6.34 4.93 2.46 0.70 1.76 1.06 O.35 0.35 0.00
t0 8 1 2 2 0 0 0 1
% 5.59 4.47 0.56 1.12 1.12 0.00 0.00 0.00 0.56
No. 50 31 22 10 t4 3 2 t t
% 6.8t 4.22 3.00 1.36 1.91 0.41
0.27 0.t4 0.14
each replication. One row plots 2t feet long having 25 plants were used for both experiments with three replications in each. Results
o
%
Total superior 59 51 24 t 34 Totalselected 271 284 t79 734 % superior 21.77 a7.95 t3.41 18.25 x Superior progenies were defined as those which differed significantly from the mean of the Tx127 • TxS08 check (x = 112.7 bu./A.) at the .01 probability level.
40 - -
0t
Range
70
80
90 100 110 120 Yield (bu./A.) for fx403 Test
130
140
150
Fig. 2. Frequency distributions for grain yield of the check populations and of the test-cross progenies of gametes sampled from the [(FYD-2) (YS-2) X (Jarvis)(Indian Chief)] population using Tx127C and Tx403 as elite lines fication was used for both experiments. I t was restricted in the sense t h a t the check (Tx403• Tx508) for the Tx403 experiment was repeated every 19th plot, and the check ( T x I 2 7 C • Tx508) for Tx127C was repeated every 20th plot. The test-crosses were conlpletely randomized in
and
Discussion
Since t h e selections were to be m a d e on t h e basis of c o m b i n i n g ability, g r ai n y i el d w as t h e m o s t i m p o r t a n t c h a r a c t e r . H o w e v e r , o b s e r v a t i o n s were also t a k e n on o t h e r c h a r a c t e r s , i n c l u d i n g d a y s to silk, r o o t l o d g i n g alld s t a l k b r e a k i n g , n u m b e r of ears p er t 0 0 plants, p e r c e n t a g e of diseased ears, a n d e x t e n t of e a r w o r m damage. T h e f r e q u e n c y d i s t r i b u t i o n s of t h e test-cross progenies f r o m t h e t h r e e g a m e t i c p o p u l a t i o n s a n d t h e single cross c o n t r o l p o p u l a t i o n s are sh o w n in F i g u r e s t , 2 an d 3- V a r i a b i l i t y w i t h i n t h e h o m o g e n e o u s cont r o l p o p u l a t i o n s is p r e s u m e d to r e p r e s e n t t h e effect of e n v i r o n m e n t , while t h e v a r i a t i o n seen in g a m e t i c test-cross p o p u l a t i o n s s t e m s f r o m b o t h g e n e t i c differences a m o n g t h e g a m e t e s s a m p l e d a n d e n v i r o n m e n t a l sources. T h e check p o p u l a t i o n p r o v i d e d a measu r e m e n t of t h e c o m b i n i n g a b i l i t y of t h e elite i n b r e d line w i t h t h e t e s t e r i n b r e d line. T h i s e s t a b l i s h e d a
50
i
-
~_ 40
-
-
-
i
-
-
I
i
i
Tesl- crogs Progenies : ~= 117.3 Tx127C 9 TxS08 : F=112.7
~zo O
a0
6O
2 70
t
------
o
N
I
J-J'J
90 100 110 120 Yietd(bu./A.) for Tx127C lest
I
130
140
150
130
140
150
I
fesl-cross Progenies : Y= 112.8 /x403 * fx508 : ~'=1188
~- 40 o
20 ~
1-70
381
Gamete Selection with an Inbred Tester
Vol. 39, No. 8
BO
go 1% 110 120 Yield (bu./A.)for Tx4O3 lest
Fig. 3. Frequency distributions for grain yield of the check poprdations and of the test-cross progenies of gametes sampled from the [(FYD-2) (YS-2) • Jarvis] population using T x I 2 7 C and Tx403 as elite lines
base for comparing the relative merits of the gametes sampled from the variable gametic sources. I t also provided an estimate of environmental variation. The check population grain yield data were analyzed separately for each experiment. For the elite line Tx403 there were 34 check plots in each replication. Test-crosses whose mean yields exceeded the check mean at the 5 % probability level were accepted as superior. Of the 67t selections tested 51 entries qualified as superior. They comprised 7.60% of the gametes sampled from the variable gametic sources. Table t shows the percentages of superior gametes in each population and the yield distributions in bu./acre for the superior progenies sampled from the three populations separately and jointly. F o r t y check-plots were used in each replication for the elite inbred line TxI27C. At the 5% level 30.33~ of the gametes tested were superior to Txt27C. Of the 734 selections tested t34 entries qualified as superior at the 10/0 probability level. These comprised 18.25% of the gametes sampled from the variable gametic sources. The percentages of superior gametes and also the frequency distribution of yield in bu./acre for these selections are shown in Table 2. This difference in the results m a y be associated with the choice of elite lines. Although not directly comparable, the two experiments were evaluated in
adjacent areas in the same year, and t h e y were extremely similar except for the elite lines used. A comparison of the test-cross distributions (Fig. t, 2 and 3) indicates rather similar yield distributions were obtained from each of the three variable gametic sources in both experiments. Thus, it would appear t h a t the principal reason for the difference in the frequencies of superior gametes is related to the higher yield and greater variability of the (Tx403 • TxS08) check population. An obvious conclusion to be drawn from this is t h a t comparisons of the frequencies of superior gametes identified in different experiments are rather meaningless, since the yield level and variability of the check populations influence these frequencies. The use of an inbred tester provides some very positive advantages for this type of selection. Since the inbred is essentially monogametic, genetic variation among test-crosses can be attributed to differences among the gametes sampled from the original populations. At the same time, the precision of these comparisons is improved b y using a single cross check. In the opinion of the authors, these advantages Outweigh the disadvantages associated with the use of an inbred tester. I t is apparent t h a t gamete selection should be considered only as a screening procedure to identify those gametes with above average potential. However, this potential can be utilized even though the superior gametes cannot be recovered in the homozygous condition. If the average gametic array of an F 1 plant exhibits significantly greater combining ability than the elite inbred line, then a conventional selfing with selection or backcrossing program should have a high probability of yielding improved forms of the original inbred line. The continued use of gamete selection through the early segregating generations would b y expected to increase the probability of Success.
Literature cited 1. GIESBRECHT, J.: Gamete selection in two early maturing corn varieties. Crop Sci. 4, 19--21 (1964). -2. LONNQUIST,J. H., and D. P. McGILL : Gamete sampling from selected zygotes in corn breeding. Agron. J. 46, 147-150 (1954). - 3. PINNELL, E. L., E. H. RINKE, and H. K. HAYES: Gamete selection for specific combining ability. In: Heterosis, pp. 378--388. Ames, Iowa: Iowa State College Press 1952. -- 4. STADLER, L. J." Gamete selection in corn breeding. J. Am. Soc. Agron. 36, 988 to 989 (1945 a). -- 5. STArtLER,L. J. : Gamete selection in corn breeding. Maize Genetics Co-operation News Letter 19, 33--40 (1945b). Dr. J. D. SMITH
Received July 15, 1969 Communicated by R. W. ALLARD
M. Z. EL-HIFNY Agronomy Department Assiut University Assiut (Egypt) M. S. AHMAD Agricultural Research Institute Dacca-15 (East Pakistan)
Associate Professor Genetics Section Department of Plant Sciences Texas A & M University College Station, Texas 77843 (USA) A. J. BOCKHOLT Assistant Professor Department of Soil and Crop Sciences Texas A & M University College Station, Texas 77843 (USA)
G a m e t e S e l e c t i o n w i t h a n I n b r e d Tester 1 M. Z. EL-HIFNY, M. S. AHMAD, J. D. SMITH and A. J. BOCKHOLT Texas A & M University, College Station, Texas Summary. A common inbred tester was used to evaluate gametes selected from three complex hybrid populations using two different inbreds as elite lines. The results support STADLER'Scontention that gamete selection is an efficient method for extracting superior gene combinations from hybrid populations. The inbred tester increases the resolving power of the method by eliminating extraneous genetic variability and by providing a homogeneous check population for comparative purposes.
Introduction The initial successes of hybrid corn breeding were obtained with inbred lines t h a t were isolated directly from open-pollinated varieties. As the hybrid corn program developed emphasis was shifted from the isolation of new inbreds to the improvement of existing inbreds. STADLER (1945a, b) proposed a modification of early testing, which he called gamete selection, as a method of obtaining improved inbred lines. The scheme involves crossing, a genetically variable population with an 'elite' inbred line and subsequently crossing individual F 1 plants to a common tester. If an inbred tester is used, as is suggested in this paper, all test-cross progenies receive essentially identical genetic contributions from the tester and from the elite inbred. Thus, genetic differences among test-cross progenies result only from differences among the gametes sampled from the variable gametic source. This allows recognition of the better gametes, but it does not permit their intact recovery. However, if the test-cross progenies are compared with the elite inbred b y tester hybrid, those which are superior to this check hybrid are presumed to have received a gamete from the variable source which was superior to the gamete contributed b y the elite inbred. Inbreeding these F 1 plants would produce modifications of the elite line which would be superior to the original inbred. STADLER suggested that gametic sampling offers a more efficient means of sampling open-pollinated varieties. PINNELL et al. (1952) used gamete selection in improving inbreds and used single crosses as specific combining ability testers. The agreement for combining ability of S O and S 1 lines was very good indicating gamete selection was an effective breeding method. LONNQUIST and MCGILL (1954) studied gametic sampling as a means of obtaining better inbred lines for specific combinations. They concluded from the Technical Article No. TA7973, Texas Agricultural Experiment Station, College Station, Texas. Project No. 1280. Parts of dissertations submitted by the first two authors in partial fulfilhnent of requirements for Ph.D. degrees.
S O and S 1 test-cross results that selection of better varietal plants as sources of germplasm to improve the inbred lines effectively increased the probability of isolation of the superior gamete combination. GIESBRECHT (t964) pointed out in his s t u d y of gametic selection that superior gametes existed in open-pollinated varieties. But he failed to observe a relationship between the yields of the F 1 plants and those of their respective F 2 and F~ progenies. He used a single cross (WD• as a tester for his study. This paper sulmnarizes the effectiveness of gamete selection, using an inbred line as tester, in two related experiments with two different elite lines using the same sources of gametes.
Materials and Methods The following complex hybrid populations were used as sources of superior gametes: A. (FYD-2 • YS-2) • Indian Chief B. (FYD-2 • YS-2) x (Jarvis • Indian Chief) C. (FYD-2 X YS-2) x Jarvis Ferguson's Yellow Dent (FYD) and Yellow Surcropper (YS) are open-pollinated Texas varieties. The FYD-2 and YS-2 parental materials are from a second cycle reciprocal recurrent selection program. Jarvis and Indian Chief are open-pollinated varieties from North Carolina. The inbred lines Tx403 and Tx127C were used as elite lines. Tx403 is an inbred selected from the backcross (Tx203 • WF9) • Tx203. The inbred line TxI27C was developed from Mosshart Yellow Dent in Texas, and it has good combining ability. However, it also has numerous agronomic defects. The inbred line TxS08, a selection from Tx325 • B37, was used as a tester for both elite lines. This tester possesses good agronomic characters and combining ability. Bulked pollen samples were collected from each of the complex hybrid populations and used to pollinate the elite lines Tx403 and TxI27C. Thus, individual F 1 plants would differ according to the genetic complement of the gamete they received from the variable gametic sources. Visually selected F1 plants were selfed, backcrossed to four inbred parent plants and test-crossed to four tester plants. The numbers of test-crossed selections from populations A, B and C were 278, t78 and t95, and 27t, 284 and t 79 for the elite lines Tx403 and Txt 27C, respectively. In t 967 the evaluation tests for the gametic sampling using Tx403 were separated from those using Tx127C. However, both were grown in the same field. A "restricted" randomized block design having a nested subclassi-
380
~[. Z. EL-HIFNY, IV[. S. AHMAD, J. D. SMITH and A.
80
,
-----
,
~
,
-i
\ 80
90
100
110
120
130
lZ,0
field (bu./A) lor 7x127C lest
80
I
I
I
t
r
I
I
Range
PopulationA PopulationB PopulationC Total
bu./acre
No.
t28.1 --130.0 t30.1 --132.0 132.1 --134.0 134.1--136.0 136.t-138.0 150 138.1 -- 140.0 140.1--142.0 142.1 -- 144.0 t44.1--146.0
5 4 3 4 t 0 o 0 t
%
No.
t.80 t.44 1.08 1.44 0.36 0.00 o.oo 0.00 0.36
2 5 1 5 2 0 2 0 o
%
No.
1.01 2.53 0.51 2.53 1.01 0.00 t.ol 0.00 0.00
5 4 3 2 1 0 t 0 0
% 2.56 2.05 t.54 t.03 0.51 0.00 o.51 0.00 0.00
No. 12 13 7 tl 4 0 3 0 t
% t.79 t.94 1.04 t.64 0.60 0.00 0.45 0.00 0.t5
Total superior 18 i7 t6 5t Total selected 278 189 195 671 % superior 6.47 8.59 8.21 7.60 x Superior progenies were defined as those which differed significantly from the mean of the Tx403 • Tx508 check (~ = 118.8 bu./A.) at the .05 probability level
Test-cross Progenies:Y=1152 A 1x403, Tx508 :~=118.8 J'X
00
Theoret. Appl. Genetics
the three variable gametic sources/or the elite inbred line Tx4o3
i
' ! 70
BOCKHOLT:
Table 1. Frequency distributions o/ superior progenies 1 selected from
l
lest-cross Progenies : Y=120.3 fx127C , fx508:7=1127
J.
Table 2. Frequency distributions o/superior progenies x selected from 70
06'0
the three variable gametic sources/or the elite inbred line Tx127C 80
90 100 110 120 'field (bu/A.) for [x403 Test
130
140
150
lZig. 1. Frequency distributions for grain yield of the check populations and of the test-cross progenies of gametes sampled from the [(FYD-2 • YS-2) • Indian Chief] population using Tx127C and Tx403 as elite lines lOO
I
I
I
]
I
Test-cross Progenies:Y=121.3 -
80
50
~
PopulationA P9pulationB PopulationC Total
bu./acre
No.
122.6--125.0 125.t --127.5 127.6--130.0 130.1--132.5 132.6--t35.0 135.1--137.5 137.6--140.0 t40.1--142.5 142.6--145.0
22 9 14 6 7 0 1 0 0
%
No.
8.t2 3.32 5.17 2.21 2.58 0.00 0.37 0.00 0.00
18 t4 7 2 5 3 1 1 0
20--70
80
[
I
90 100 110 120 Yield { bu./A) for Tx127C Test
130
140
150
I
I
Test-cross Progenies: 7= 114.5 fx403./'x508 : ~-= 118.8
60
A
40
/J \
20 0
r
60
No.
6.34 4.93 2.46 0.70 1.76 1.06 O.35 0.35 0.00
t0 8 1 2 2 0 0 0 1
% 5.59 4.47 0.56 1.12 1.12 0.00 0.00 0.00 0.56
No. 50 31 22 10 t4 3 2 t t
% 6.8t 4.22 3.00 1.36 1.91 0.41
0.27 0.t4 0.14
each replication. One row plots 2t feet long having 25 plants were used for both experiments with three replications in each. Results
o
%
Total superior 59 51 24 t 34 Totalselected 271 284 t79 734 % superior 21.77 a7.95 t3.41 18.25 x Superior progenies were defined as those which differed significantly from the mean of the Tx127 • TxS08 check (x = 112.7 bu./A.) at the .01 probability level.
40 - -
0t
Range
70
80
90 100 110 120 Yield (bu./A.) for fx403 Test
130
140
150
Fig. 2. Frequency distributions for grain yield of the check populations and of the test-cross progenies of gametes sampled from the [(FYD-2) (YS-2) X (Jarvis)(Indian Chief)] population using Tx127C and Tx403 as elite lines fication was used for both experiments. I t was restricted in the sense t h a t the check (Tx403• Tx508) for the Tx403 experiment was repeated every 19th plot, and the check ( T x I 2 7 C • Tx508) for Tx127C was repeated every 20th plot. The test-crosses were conlpletely randomized in
and
Discussion
Since t h e selections were to be m a d e on t h e basis of c o m b i n i n g ability, g r ai n y i el d w as t h e m o s t i m p o r t a n t c h a r a c t e r . H o w e v e r , o b s e r v a t i o n s were also t a k e n on o t h e r c h a r a c t e r s , i n c l u d i n g d a y s to silk, r o o t l o d g i n g alld s t a l k b r e a k i n g , n u m b e r of ears p er t 0 0 plants, p e r c e n t a g e of diseased ears, a n d e x t e n t of e a r w o r m damage. T h e f r e q u e n c y d i s t r i b u t i o n s of t h e test-cross progenies f r o m t h e t h r e e g a m e t i c p o p u l a t i o n s a n d t h e single cross c o n t r o l p o p u l a t i o n s are sh o w n in F i g u r e s t , 2 an d 3- V a r i a b i l i t y w i t h i n t h e h o m o g e n e o u s cont r o l p o p u l a t i o n s is p r e s u m e d to r e p r e s e n t t h e effect of e n v i r o n m e n t , while t h e v a r i a t i o n seen in g a m e t i c test-cross p o p u l a t i o n s s t e m s f r o m b o t h g e n e t i c differences a m o n g t h e g a m e t e s s a m p l e d a n d e n v i r o n m e n t a l sources. T h e check p o p u l a t i o n p r o v i d e d a measu r e m e n t of t h e c o m b i n i n g a b i l i t y of t h e elite i n b r e d line w i t h t h e t e s t e r i n b r e d line. T h i s e s t a b l i s h e d a
50
i
-
~_ 40
-
-
-
i
-
-
I
i
i
Tesl- crogs Progenies : ~= 117.3 Tx127C 9 TxS08 : F=112.7
~zo O
a0
6O
2 70
t
------
o
N
I
J-J'J
90 100 110 120 Yietd(bu./A.) for Tx127C lest
I
130
140
150
130
140
150
I
fesl-cross Progenies : Y= 112.8 /x403 * fx508 : ~'=1188
~- 40 o
20 ~
1-70
381
Gamete Selection with an Inbred Tester
Vol. 39, No. 8
BO
go 1% 110 120 Yield (bu./A.)for Tx4O3 lest
Fig. 3. Frequency distributions for grain yield of the check poprdations and of the test-cross progenies of gametes sampled from the [(FYD-2) (YS-2) • Jarvis] population using T x I 2 7 C and Tx403 as elite lines
base for comparing the relative merits of the gametes sampled from the variable gametic sources. I t also provided an estimate of environmental variation. The check population grain yield data were analyzed separately for each experiment. For the elite line Tx403 there were 34 check plots in each replication. Test-crosses whose mean yields exceeded the check mean at the 5 % probability level were accepted as superior. Of the 67t selections tested 51 entries qualified as superior. They comprised 7.60% of the gametes sampled from the variable gametic sources. Table t shows the percentages of superior gametes in each population and the yield distributions in bu./acre for the superior progenies sampled from the three populations separately and jointly. F o r t y check-plots were used in each replication for the elite inbred line TxI27C. At the 5% level 30.33~ of the gametes tested were superior to Txt27C. Of the 734 selections tested t34 entries qualified as superior at the 10/0 probability level. These comprised 18.25% of the gametes sampled from the variable gametic sources. The percentages of superior gametes and also the frequency distribution of yield in bu./acre for these selections are shown in Table 2. This difference in the results m a y be associated with the choice of elite lines. Although not directly comparable, the two experiments were evaluated in
adjacent areas in the same year, and t h e y were extremely similar except for the elite lines used. A comparison of the test-cross distributions (Fig. t, 2 and 3) indicates rather similar yield distributions were obtained from each of the three variable gametic sources in both experiments. Thus, it would appear t h a t the principal reason for the difference in the frequencies of superior gametes is related to the higher yield and greater variability of the (Tx403 • TxS08) check population. An obvious conclusion to be drawn from this is t h a t comparisons of the frequencies of superior gametes identified in different experiments are rather meaningless, since the yield level and variability of the check populations influence these frequencies. The use of an inbred tester provides some very positive advantages for this type of selection. Since the inbred is essentially monogametic, genetic variation among test-crosses can be attributed to differences among the gametes sampled from the original populations. At the same time, the precision of these comparisons is improved b y using a single cross check. In the opinion of the authors, these advantages Outweigh the disadvantages associated with the use of an inbred tester. I t is apparent t h a t gamete selection should be considered only as a screening procedure to identify those gametes with above average potential. However, this potential can be utilized even though the superior gametes cannot be recovered in the homozygous condition. If the average gametic array of an F 1 plant exhibits significantly greater combining ability than the elite inbred line, then a conventional selfing with selection or backcrossing program should have a high probability of yielding improved forms of the original inbred line. The continued use of gamete selection through the early segregating generations would b y expected to increase the probability of Success.
Literature cited 1. GIESBRECHT, J.: Gamete selection in two early maturing corn varieties. Crop Sci. 4, 19--21 (1964). -2. LONNQUIST,J. H., and D. P. McGILL : Gamete sampling from selected zygotes in corn breeding. Agron. J. 46, 147-150 (1954). - 3. PINNELL, E. L., E. H. RINKE, and H. K. HAYES: Gamete selection for specific combining ability. In: Heterosis, pp. 378--388. Ames, Iowa: Iowa State College Press 1952. -- 4. STADLER, L. J." Gamete selection in corn breeding. J. Am. Soc. Agron. 36, 988 to 989 (1945 a). -- 5. STArtLER,L. J. : Gamete selection in corn breeding. Maize Genetics Co-operation News Letter 19, 33--40 (1945b). Dr. J. D. SMITH
Received July 15, 1969 Communicated by R. W. ALLARD
M. Z. EL-HIFNY Agronomy Department Assiut University Assiut (Egypt) M. S. AHMAD Agricultural Research Institute Dacca-15 (East Pakistan)
Associate Professor Genetics Section Department of Plant Sciences Texas A & M University College Station, Texas 77843 (USA) A. J. BOCKHOLT Assistant Professor Department of Soil and Crop Sciences Texas A & M University College Station, Texas 77843 (USA)
